Coating of substances, e.g., glass surfaces, through sputtering/cathode sputtering in plasma processes, in a reactive and also conventional manner, is e.g., known from architecture glass coating. Towards this end, a current of voltage source generates a plasma that removes material from a target, which have been deposited on a substrate, e.g., a glass panel. Prior to depositing, the atoms may bind to gas atoms or gas molecules in a reactive process, depending on the desired coating.
Medium frequency (MF) generators are usually operated at a frequency of 10 to 500 kHz, and are often used, e.g., for reactive processes. The output voltage of MF generators is usually supplied to two electrodes in a plasma process chamber, and the two electrodes alternately work as a cathode and an anode and are each connected to one target. MF generators can operate at a freely oscillating frequency, or can operate at a fixed frequency.
In particular, in reactive processes, MF generators frequently also produce sparkovers, so-called micro arcs, that often automatically extinguish during the next voltage reversal or at least after a few periods. However, arcs of a higher energy and longer duration may also occur. Arcs are often detected by checking the output voltage of the generator for a voltage drop or by checking the output current of the generator for a current increase. An arc may alternatively be recognized by the difference between the currents to the individual electrodes. In conventional generators, the user can adjust an arc detection limit value. The effective values of the current and the voltage are measured for the detection. In such a measurement, the values of the voltage and current are integrated over the period in order to prevent zero crossings from being detected as voltage drops. This type of arc detection is therefore normally much slower than the duration of a half-wave of the MF output voltage, and therefore can be longer than 40 μs.
MF generators that are used in semiconductor production processes, e.g., in flat panel display (FPD) production processes, should be able to detect arc within a few μs or even less than one μs.
In the arc detection method disclosed in DE 43 26 100 A1, each half-wave of the medium frequency signal of the medium frequency generator is divided into a plurality of time periods, the values of current and voltage being measured for a predetermined time period for forming an actual value signal, and input into a ground-free measuring device. The measuring device is embedded in a loop whose master station is located in the control unit in the generator, and upon occurrence of an arc, the generator is disabled through a connecting line that connects the measuring device to the generator.
U.S. Pat. No. 6,420,863 B1 proposes measuring values of at least one discharge current or one discharge voltage for each half-wave. The difference between the measured values of a first and a second half-wave is established. The difference is compared with predetermined tolerances and, when the tolerances are exceeded, the power supply is reduced.